Emergency departments (EDs) in the United States are in a state of crisis (Kellerman, 2006; IOM, 2007). An epidemic of ED overcrowding, boarding of patients awaiting inpatient beds, and ambulance diversion threatens the very integrity of the healthcare safety net. While some ED encounters are unavoidable, there is a set of conditions known as ambulatory care sensitive conditions (ACSC), for which access to appropriate primary care is thought to prevent the need for treatment in an acute care setting. Of all ED visits, approximately 8.4% are for ACSC (Johnson et al., 2012). Avoidable ED visits and resultant hospital admissions place enormous strain on the healthcare delivery system (AHRQ, 2011). Reducing avoidable ED encounters has potential to reduce cost, improve quality and enhance efficiency (AHRQ, 2011). Though rates of hospitalizations for ACSC are often used as regional measures of access to and quality of primary care, there is controversy among policy experts regarding the association between provider density and broader healthcare utilization. Some experts argue that increased PCP availability would reduce the rate of ED encounters for ACSC. Others contend that increasing provider supply could result in increased rates of healthcare utilization across all venues of care as the effects of improved chronic disease management might be offset by over-diagnosis, over-treatment, and iatrogenic complications. We propose a study to further investigate the critical association between PCP density and rates of ED encounters and hospitalizations for ACSC using data from the California Office of State Planning and Development. First, we will establish the impact of PCP density on rates of ED encounters and hospitalizations for ACSC using multivariate analysis and a county-level fixed-effects model. A potential causal relationship between will be explored by examining the effect of changes in PCP density over time against changes in encounters for ACSC over time. We hypothesize that rates of encounters for ACSC will increase in counties experiencing a decline in PCP density over time, and decrease among counties experiencing an increase in PCP density over time. Next, we will identify how the interaction between encounters for ACSC and PCP density varies by payor source, and among AHRQ priority populations. We hypothesize significant variation in the relationship between rates of ED encounters and hospitalizations for ACSC and PCP density among AHRQ priority populations, and that populations with strong correlations may benefit from interventions to increase PCP density in underserved counties while those with no correlation identified will require alternate interventions to improve access to care. Finally, we will estimate costs associated with ED encounters and hospitalizations by applying cost-to-charge ratios to charge data for ACSC encounters. We hypothesize that costs associated with ED encounters and hospitalizations for ACSC will be greatest in the counties with lowest PCP densities, and that findings may help to justify improved incentives for PCPs to practice in underserved areas.